Waterborne vessels are typically maneuvered using a conventional rudder located at or near the stern of the ship. A conventional rudder is a substantially planar member that is rotated around an axis perpendicular, or nearly perpendicular, to the surface of the water. Ride quality, namely minimization of undesirable vessel pitch and roll, is provided by having one or more of the following: a small waterplane area ship, control surfaces such as canards, stabilizers, and/or foils, an automatic control system, and other active devices. Canards 2 and stabilizers 4 (shown in FIG. 1) are substantially planar members rotated about an axis parallel, or nearly parallel, to the surface of the water. Canards are typically located forward of the center of gravity of the vessel. Stabilizers are typically located aft of the center of gravity, while foils are located forward or aft of the center of gravity. Water flowing over a control surface creates a lift force normal to the direction of flow and a drag force parallel to the direction of flow acting at the center of pressure. The magnitude of the lift and drag force is proportional to the size of the control surface and the inflow velocity over the surface. When a control surface is rotated about its axis, the magnitude and direction of this hydrodynamic force changes. In the case of a rudder, this hydrodynamic force applied at the stern of the ship creates a turning moment around the center of mass, turning the vessel in the direction of the moment. In the case of canards, stabilizers, or foils, this hydrodynamic force acting on any or all the control surfaces creates a pitching and/or rolling moment around the center of mass, rotating the vessel in the direction of the moment.
Waterborne vessels that require good ride quality and high maneuverability, at all speeds, will most likely have a small waterplane, incorporate canards, stabilizers, and/or foils for ride control, and rudder(s) for maneuvering. However, incorporating all these control surfaces on a ship can have an adverse affect on the top speed due to the drag associated with each control surface.
Maneuvering: When a ship is executing a turn, a centrifugal force is generated, which acts horizontally through the center of gravity. The magnitude of the centrifugal force is proportional to the weight of the vessel, the square of the vessel velocity and the radius of turn. This centrifugal force is balanced by a horizontal water pressure acting on the underwater portion of the ship, as illustrated in FIG. 2. This heeling moment, which increases with the square of the forward speed of the vessel, tends to roll the vessel in a direction opposite to the direction of a steady turn. The ship will heel until the moment of the ship's weight and buoyancy, the righting moment, equals that of the centrifugal force and the water pressure. The righting moment is generated by the shifting of the center of buoyancy of the vessel opposite the direction of the turn, as shown in FIG. 2. Ships with large waterplane areas resist this heeling moment better than ships with small waterplane areas, reducing the angle of inclination or roll angle. However, ride quality is compromised. Small waterplane area vessels will have superior ride quality compared to large waterplane area ships but tend to experience greater roll angles during a turn because of their reduced waterplane area. Although, conventional rudders, and some canards and stabilizers, known in the art, will provide a moment that resists the heeling moment, they typically do not provide the required hydrodynamic force sufficient to prevent the ship from rolling out of the turn. If the rudder is large enough, or separated sufficiently far from the ship's center of gravity a moment sufficient to counter the heeling moment can result in a level turn or roll into the turn. Unfortunately, neither of these choices is desirable due to the excessive drag or possible extensive draft from the large rudder.
Ride quality: When a ship experiences waves in a seaway, hydrodynamic forces, caused by surface effects and pressure distributions along the hull, cause undesirable pitching and rolling moments on the ship. Small waterplane area ships are more resilient to these undesirable motions than large waterplane area ships; however they still experience some level of roll and pitch motions. A motion control system utilizing canards, stabilizers, and/or foils are often incorporated in a ship design to prevent these unwanted motions. Clearly, the size of the control surfaces and the separation distance from the center of gravity have an impact on the ability to resist these motions.
Having separate control surfaces for ride control, such as canards and stabilizers, and rudders for turning can affect the vessel top speed and limit the choices to the operator. It has been a long felt desire by naval architects and marine engineers to design a ship with superior ride quality and high maneuverability, while hot compromising the vessel top speed. These conflicting requirements continually pose a challenge to the designers.
Clearly, then, there is a long felt need for a control surface or combination of control surfaces that enable a vessel to be steered along a desired heading, while also minimizing rolling and pitching moments. Further, there is a long felt need for a vessel able to execute a turn at any desired speed with the vessel rolling into the turn. Finally, there is a need to implement the above capabilities without imposing excessive drag on the vessel.